EEPROMs (Electrically Erasable and Programmable Read Only Memories) have been widely used as electrically programmable/erasable nonvolatile semiconductor memory devices. These memory devices (memories) typified by flash memories, which are widely used today, have an electrically conductive floating gate electrode or trapping dielectric film surrounded by oxide films under a gate electrode of a MOS (Metal Oxide Semiconductor) transistor. A charge trapping state in the floating gate or in a trapping characteristic dielectric film is used as memory information, and it is read as a threshold value of the transistor.
The trapping dielectric film is a dielectric film capable of trapping charges, and a silicon nitride film is referred to as an example thereof. The threshold value of the MOS transistor is shifted by injection/discharge of charges into or from such a charge trapping region to operate it as a memory element. A nonvolatile memory that uses such a silicon nitride film as the charge trapping region is called a MONOS (Metal Oxide Nitride Oxide Semiconductor) type memory, and is excellent in the reliability of data retention as compared with the electrically conductive floating gate film since charges are discretely trapped. Moreover, since it is excellent in the reliability of data retention, it has advantages in which, for example, oxide films over and under the silicon nitride film can be made to be thinner, so that low voltage program/erase operations can be performed.
The silicon nitride film used in the charge trapping region of the above-described MONOS type memory always contains hydrogen. It is known that a contained hydrogen concentration is approximately 3×1021 cm−3 in the case where the film is formed by generally-used Low Pressure Chemical Vapor Deposition (LPCVD). (for example, see Physical Review B, Vol. 48, pp 5444, 1993, Non-Patent Document 1). The hydrogen is present in the form of a bond between silicon and hydrogen (Si—H bond) or bond between nitrogen and hydrogen (N—H bond). Generally, the number of N—H bonds is larger than that of Si—H bonds (for example, see Journal of the Electrochemical Society, Vol. 124, pp. 909, 1977, Non-Patent Document 2).
Concerning the hydrogen of the silicon nitride film used in the charge trapping region of the MONOS type memory, several proposals for improving the data retention characteristics by reducing the density of Si—H bonds have been made. In Japanese Patent Application Laid-open Publication No. 2006-128593 (Patent Document 1), the silicon nitride film is formed by Chemical Vapor Deposition (CVD) under a condition in which a flow ratio of dichlorosilane (DCS: SiCl2H2)/ammonia (NH3) is 0.1 or less, so that the density of Si—H bonds is set at 1×1021 cm−3 or less. As a result, the trap density in the silicon nitride film is reduced, the charges in the silicon nitride film are not readily moved, and therefore, the data retention characteristics can be improved.
In Japanese Patent Application Laid-open Publication No. 2004-356562 (Patent Document 2), the density of the Si—H bonds in the silicon nitride film is made to be 1×1020 cm−3 or less by employing Atomic Layer Deposition (ALD). As a result, shallow traps in the silicon nitride film are reduced, the charges in the silicon nitride film are not readily moved, and the data retention characteristics can be improved.
The reason why the Si—H bond is attracted instead of the N—H bond higher in density than the Si—H bond being present in the silicon nitride film is that bonding energy of the Si—H bond is smaller, and therefore the bond is readily broken by energy such as a thermal load during manufacturing processes.
As a method of forming the silicon nitride film with a low hydrogen concentration including the N—H bond, there is a method in which the film is formed by using a gas containing no hydrogen. In Japanese Patent Application Laid-open Publication No. 2002-203917 (Patent Document 3), a method in which the silicon nitride film is formed by using silicon tetrachloride SiCl4 and ionized nitrogen in a plasma state is disclosed.